1. Field of the Invention
This invention relates to magnetic resonance imaging (MRI). More specifically, it relates to a four-ring birdcage coil having at least one tuning ring for double resonance MRI.
2. Description of the Prior Art
Birdcage coils for magnetic resonance apparatuses are generally known. In magnetic resonance systems, birdcage coils are used as transmission elements for excitation of magnetic resonances. Furthermore, they are also used as whole-body reception elements for magnetic resonance systems as well as, in some cases, local coils (for example as head coils).
A shortcoming of some conventional double-tuned birdcage coils, including those described by Murphy-Boesch et al. J. Mag. Res., vol. 103, pp. 103-114 (1994), is the difficulty in tuning the resonant frequencies. Conventional double-tuned birdcage coils have dependent tuning. An example of a double-tuned birdcage coil is described in U.S. Pat. No. 4,916,418, where two different sets of capacitors are in the path of both the low and high frequency currents, and adjusting one set of capacitances affects both low and high resonant frequencies. Thus, the double-tuned birdcage coils are dependently tuned such that more than one set of capacitances on the coil must be adjusted to tune either the lower or higher frequency, and significant amounts of time may be required for an iterative tuning procedure.
U.S. Pat. No. 6,100,694 uses a combination of low-pass and high-pass configuration to achieve double resonance operation. Because the high-pass configuration requires more capacitance, it is difficult to use distributed capacitance in the design and difficult to construct a double resonance coil based on the modification of a single resonance coil by the introduction of coupling rings.
Thus, what is needed is a double-tuned birdcage coil that may be conveniently tuned to cover a large frequency range. What is also needed is a multiply tuned birdcage coil that has independent tuning on each channel. Moreover, what is needed is a way to easily construct a double resonance coil based on the modification of a single resonance coil.